1. Field of the Invention
The present invention relates to an alignment apparatus provided with a sensor capable of detecting the position of an object at a resolution of the order of nanometers and, more particularly, to an aligment apparatus suitable for use with an exposure system for manufacturing semiconductor devices.
2. Related Background Art
In the field of optical lithography, the wavelength of a laser source for use in printing the pattern of a mask on a wafer, that is, an exposure wavelength, has recently been made shorter and shorter. With the development of a projection lens having an improved numerical aperture (N.A.), the mimimim resolvable linewidth has reached the order of submicrons, for example, 0.5-0.7 .mu.m. If a pattern which requires such a high resolution is to be transferred in practice, it is necessary that the matching accuracy of a pattern formed on a wafer with respect to a new mask pattern image be made high so as to meet the aforesaid resolution. As one method of improving the matching accuracy, it is proposed to improve the accuracy with which an alignment mark is detected, that is, resolution.
A number of techniques have recently been published for detecting the position of a mark at a resolution of the order of nanometers. One technique utilizes a method which comprises the steps of illuminating a diffraction grating formed on a substrate, such as a wafer, with a pair of coherent collimated beams in two different directions, producing one-dimensional interference fringes on the diffraction grating, and photo-electrically detecting the intensity of a diffracted beam or an interference beam which is produced at the diffraction grating as a result of illumination with the interference fringes. A heterodyne method is also known in which a pair of coherent beams having slightly different frequencies illuminate a diffraction grating in a manner similar to the above-described one and an interference beam to be photo-electrically detected is intensity-modulated at a beat frequency.
More specifically, the heterodyne method is to detect the positional offset within .+-.1/4 pitch of the diffaraction mark by obtaining the phase difference within .+-.180.degree. between the photo-electric signal of a measurement-purpose interference beat beam coming from the diffraction mark on the wafer and the photo-electric signal of a reference-purpose interference beat beam. The photo-electric signal of the reference-purpose interference beat beam is separately produced from the aforesaid pair of coherent beams.
For example, if the diffraction pitch P of the diffraction grating mark on the wafer is 2 .mu.m, that is, the diffraction pitch P includes a 1-.mu.m line and a 1-.mu.m space, the resolution of phase-difference measurement is approximately 1.5.degree.. The resolution of measurement of positional offset is: EQU P/4.multidot.(0.5.degree./180.degree.).apprxeq.0.0014 .mu.m=1.4 nm
This resolution is extremely high for an alignment sensor for detecting optical information from the diffraction mark. With the current electronic techniques, it is of course possible to accommodate the resolution and accuracy of the measurement of phase difference within 0.1.degree.. Accordingly, if a more accurate phase-difference detecting circuit or arithmethic software is employed, a fivefold resolution, that is, 0.28 nm, can be easily achieved. If a positional offset detecting system utilizing the aforesaid interference fringes is incorportated as an alignment system, such as a proximity aligner, a stepper, a mirror projection arrangment, a step-scan aligner or the like, the matching accuracy is expected to improve at least one digit with respect to conventional alignment systems.
One example in which such a heterodyne interference alignment system is incorporated in a projection exposure system, that is, a stepper, is disclosed in Japanese Patent Laid-Open No. 63-283129, which corresponds to U.S. Ser. No. 536,939 filed Jun. 12, 1990 (now U.S. Pat. No. 5,004,348 issued Apr. 2, 1991), which is a continuation application of U.S. Ser. No. 469,713 filed Jan. 24, 1990 (now abandoned), which is a continuation application of U.S. Ser. No. 192,784 filed May 10, 1988 (now abandoned). The disclosed example utilizes the various contrivances of more effectively using the heterodyne alignment system within a stepper. The primary features of the disclosed art are: (1) that a reticle mark and the diffraction grating of a wafer can be simultaneously detected by means of non-exposure wavelengths in a through-the-reticle (TTR) system: and (2) that it is possible to consistently monitor the positional offset of a mark by the heterodyne alignment system even during the exposure of the wafer by means of a dichroic mirror disposed above the reticle.
However, the aforesaid prior art has the following problems.
The aforesaid measurement-purpose interference beat beam and reference-purpose interference beat beam are produced from the same laser beam. That is to say, a pair of coherent beams is produced from the aforesaid laser beam. These two coherent beams respectively reach the diffraction grating of the wafer by way of a reticle supported on the stepper and a projection lens assembly. The measurement-purpose beat beam coming from the diffraction grating of the wafer returns through the projection lens assembly and the reticle, and is photo-electrically detected to thereby produce the photo-electric signal of the measurement-purpose interference beat beam. In the meantime, each of the aforesaid coherent beams is partially separated, and the separated components are transmitted to a detector by way of a reference diffraction grating and are photo-electrically detected to thereby produce the photo-electric signal of the reference-purpose interference beat beam.
More specifically, a beam transmitting path for the pair of beams which results in the reference-purpose beat beams completely differs from a beam transmitting path for the pair of beams which results in the measurement-purpose beat beam. In addition, the optical path length travelled by the pair of beams which illuminate the wafer exceeds 100 cm, even when measured from the branch point between a measurement-purpose beam receiving system and a reference-purpose beam receiving system. In particular, when the fluctuation of air occurs midway along the optical path from the reticle to the wafer, the interference fringes formed on the wafer are influenced, thus resulting in a lowering in the accuracy of detection of positional offset, imperfect alignment, or the like.